The invention is based on a priority application EP 02360221.2 which is hereby incorporated by reference.
The invention relates to a cascaded Raman laser comprising a pump radiation source emitting at a pump wavelength λp, an input section and an output section made of an optical medium, each section comprising wavelength selectors for wavelengths λ1, λ2, . . . , λn−k, where n≧3, λp<λ1<λ2< . . . <λn−1<λn and λn−k+1, λn−k+2, . . . , λn being k≧1 emitting wavelengths of the laser, and an intracavity section that is made of a non-linear optical medium, has a zero-dispersion wavelength λ0 and is disposed between the input and the output section.
Such lasers are generally known in the art, for example from U.S. Pat. No. 5,323,404.
Raman lasers typically comprise a pump source, usually a continuos wave (CW) laser, and a length of an non-linear optical medium, for example an optical fiber. Two reflectors having the same peak reflectivity are spaced apart on the optical fiber so as to form an optical laser cavity. The underlying physical principle of such lasers is the effect of spontaneous Raman scattering. This is a non-linear optical process that only occurs at high optical intensities and involves coupling of light propagating through the non-linear medium to vibrational modes of the medium, and re-radiation of the light at a different wavelength. Re-radiated light upshifted in wavelength is commonly referred to as a Stokes line, whereas light downshifted in wavelength is referred to as an Anti-Stokes line. Raman lasers are typically used an a configuration in which pump light is upshifted in wavelength. When a silica fiber is used as the non-linear medium, the strongest Raman scattering (maximum Raman gain) occurs at a frequency shift of about 13.2 THz, corresponding to a wavelength shift of about 50–100 nm for pump wavelengths between about 1 and 1.5 μm.
In Raman lasers that are configured as ring lasers, the non-linear optical fiber is closed by a coupler or an optical circulator so that a fiber loop is obtained. The reflectors are then usually replaced by optical filters, for example Fabry-Perot-Filters, having a specified passband center wavelength. The generic term “wavelength selectors” will henceforth be used for designating reflectors, filters or other means that are used to define optical resonators in Raman lasers.
A “cascaded” Raman laser is a Raman laser that has, in addition to an optical cavity for radiation of an emission wavelength λn, at least one further optical cavity for radiation of wavelength λn−1<λn, where n≧2. In such cascaded Raman lasers radiation undergoes more than one Stokes transitions so that it is subsequently upshifted in wavelength. If radiation with more than one wavelength is coupled out of the laser, such a laser is referred to as a multi-wavelength Raman laser.
Raman lasers are often used as pump lasers for Raman amplifiers at 1310 or 1550 nm, or as 1480 nm pump lasers for remotely pumped erbium fiber amplifiers in repeaterless optical fiber communication systems. Uses for other purposes at other wavelengths are possible and contemplated.
One of the key properties of cascaded Raman lasers is the conversion efficiency which is defined as the ratio between output power of the laser and optical pump power at the input side. Another key property is the threshold pump power that has to be exceeded for generating a substantial optical output power, i.e. an output power of at least some mW. With pump powers below the threshold, there is only an insignificant optical output power in the order of several μW.
A Raman laser having a low pump threshold is desirable in many respects. For example, it should allow to generate a low but stable output power. Although in many applications high output powers are an essential feature, there are other applications which require such low and stable output powers.